Interpretive Summary: Toxoplasma gondii is a single-celled parasite of all warm-blooded hosts worldwide. It causes mental retardation and loss of vision in children, and abortion in livestock. Cats are the main reservoir of T. gondii because they are the only hosts that can excrete the resistant stage (oocyst) of the parasite in the feces. Humans become infected by eating under cooked meat from infected animals and food and water contaminated with oocysts. In the present study, we found a very high prevalence of Toxoplasma in deer shot around human dwellings near the Cleveland Zoo Park, compared with low prevalence in wild deer from Minnesota. Because deer are vegetarians, the ingestion of contaminated food and water is the main mode of transmission of this parasite. The high prevalence in Cleveland Zoo Park suggests that this urban environment is highly contaminated with oocysts of this zoonotic parasite, and offers an opportunity to further study epidemiology of this disease. The results will be of interest to biologists, epidemiologists, and parasitologists.

Technical Abstract:
The prevalence of Toxoplasma gondii in white tailed deer (WTD) in the USA is high, but little is known of the epidemiology of toxoplasmosis in this host. In the present study, we compared T. gondii seroprevalence from 531 WTD collected in 2012 and 2013 from a Metropolitan Park in Ohio, and and 485 WTD deer shot in Minnesota during 2008, 2009, and 2010. Serum samples were tested for antibodies to T. gondii by the modified agglutination test (cut-off titer, 25). Additionally, myocardial samples from 123 seropositive WTD from Ohio were digested in pepsin and the digests were bioassayed for the isolation of T. gondii. Furthermore, to estimate transplacental rate of transmission, brains from 155 fetuses (included twins) from 148 deer from Minnesota were bioassayed in mice for the isolation of viable T. gondii. Seroprevalence of T. gondii varied with the year of collection, geography, and the age of deer. Of the Ohio deer sampled in 2012 and 2013 seroprevalences for the two years were similar (73.4% and 74.4%, respectively); remarkably 103 (60.9%) of 169 deer of <1 year of age were seropositive. Of the Minnesota deer, seroprevalence was lowest for the year 2008 (14.8%, 26/175) versus 2009 (27.2%, 58/213), and 2010 (23.7%, 23/97), thought to be related to environmental temperatures. Viable T. gondii was isolated in mice from the myocardium of four WTD from Ohio, and brain of one WTD fetus from Minnesota. Tachyzoites from infected mouse tissues were further propagated in cell culture. The DNA isolated from culture-derived tachyzoites of these five T. gondii isolates was characterized using 11 PCR-RFLP markers (SAG1, 5’- and 3’-SAG2, alt.SAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1 and Apico). Four genotypes were found, including ToxoDB genotype #1 (Type II), #2 (Type III), #3 (Type II variant) and #146. Results indicate fluctuating seroprevalence, probably related to weather and warrant further epidemiological studies.